In the area of nuclear magnetic resonance, the need for apparatus concurrently sensitive to non-adjacent spectral regions is encountered in several contexts. One common example occurs where a sample is irradiated at some (high) frequency for one purpose while the same sample is concurrently irradiated at another (low) frequency for some other purpose. This is typical of a decoupling experiment wherein, for example, carbon 13-hydrogen chemical bonds are decoupled while separately exciting the carbon 13 resonance concurrently.
One variation of such a double-tuned arrangement is the need for concurrent excitation and observation of chemically distinct samples where one such sample is a control employed for instrumental purposes such as the establishment of a field frequency lock, while the second sample is under study. One example of this arrangement is to be found in U.S. Pat. No. 3,434,043, commonly assigned. Another similar circumstance is the desire to concurrently excite selected different nuclei for acquisition of corresponding spectral response.
A double-tuned circuit ordinarily utilizes a single inductor common to two resonant circuits. Each sub-circuit in such an arrangement is separately tuned and impedance matched to its respective rf source (or sink). In one class of double-tuned arrangement it is necessary for an isolation element to be inserted between high frequency and low frequency sources. This permits concurrent excitation from respective rf sources. Double-tuned circuits are known which employ a transmission line of length .lambda./4 (at the high frequency) to provide such isolation. For an example of such an arrangement see Stoll, Vega, and Vaughan, Rev. Sci. Inst., v. 48, pp. 800-803 (1977). Balanced circuits exhibiting electrical symmetry are also known for the purpose of supporting double-tuned apparatus. Such circuits, exhibit among other properties, the virtue that a symmetry plane (or other surface) is defined which has a property of electrical neutrality, which is to say, a virtual ground.
Inductive elements in rf probe circuits are known to include "split inductors" such as taught in the work of Alderman and Grant, J. Mag. Res., v. 36, pp. 447-451 (1979).
An example of a balanced double-tuned circuit with split inductors and capacitances for NMR observe coils is to be found in U.S. Pat. No. 4,833,412. A double-tuned balanced circuit using lumped elements for a birdcage geometry is described in U.S. Pat. No. 4,916,418.
A saddle coil for multiple tuned response is realized by providing a segmented sequence of reactances wherein each such reactance includes alternate rf current paths exhibiting widely distinct impedance characteristic of the respective resonant frequencies. This work is described in U.S. Ser. No. 07/603,966, commonly assigned.
The present invention is motivated by a desire to observe C.sup.13 NMR signals at about 50 MHz while concurrently irradiating the sample with decoupling radiation at about 200 MHz.
The resonant frequencies in hydrogen and C.sup.13 are almost exactly in the ratio of 4 to 1. The ratio of the LC (inductance capacitance) product for a simple tuned circuit resonant at the high frequency carbon resonance to the LC product for a similar circuit resonant at the low frequency (e.g. same magnetic field) is about 16. In one common approach to double-tuned circuits, the inductance L is common to both resonant sub-circuits forming the double-tuned resonant circuit. Thus, the capacitance ratio in such respective sub-circuits will also be 16.
In the present invention the double-tuned character of the circuit is realized with two inductors (which could comprise saddle coils or surface coils) which are in parallel for the high frequency sub-circuit, but which form a series combination for the low frequency sub-circuit. The effective inductance ratio in this case is thus 4:1 (200 MHz:50 MHz), reducing the required capacitance ratio from 16:1 to 4:1.